Strapped multicavity magnetron



y 9, 1953 P. D. CROUT 2,639,403

STRAPPED MULTICAVITY MAGNETRON Filed Feb. 27, 1945 amp.-

INVENTOK.

PRESCOTT D. GROUT ATTORNEY Patented May 19, 1953 Application February 27, 19215,tetamiaotir*" 'ifhis invention relates in general to magnetron oscillators; and more particularly to multiresohater magnetron oscillators operating in such a manner that the frequency of oscillations is dependent in part upon the transit time of the electrons between cathode and anode.

According to conventional magnetron construction, the tube comprise a cylindrical cathode and a plurality of cavity resonators which are part of the anode structure and the long axes of whichare parallel to the cathode. These cavities are dispersed symmetrically about the oathode, and are directly coupledto the c'athodeanode space by radially disposed slots, the holes and; the slot forming a well known hole-andslot anode structure which is placed in strong magnetic field parallel tothe axis of magnetron. A coaxial line or a wave guide is usually electromagnetically coupled to one of the cavitiesin, orderto extract radio frequency energy from the tube. When the anode is placed at a high potential with respect to the cathode, the orbital paths of the electrons within the cathode-anode space produce'e'lectrical oscillations in the cavity resoa s-fi 7 f 'When the above-described type of operation is used, the magnetron tends to oscillate at any given instant in one of various possible modes.

E achmode has associated with it a particular electrical angle between adjacent cavity resonators, and 'a'particular field configuration within the cavities. The mode in which a magnetron will oscillate dependsjon the plate voltage, the

voltage 'aveform of the input, the magnetic field strength and thephysical constants of the tube.

It often happens that two or more modes are quite close to one another as far as small changes in; the above-mentioned variables are concerned. Thus 'a magnetron may pass through several modes if its plate voltage is varied over wide limits, a is often thecase: 'In order to increase the separation of these modes, and thus increase the stability of operation of 'the tube, the normally-small mutual coupling between alternate cavities is increased by some form of conductors called strapswhich electrically connect similar areas of alternate resonator segment'a their similar "areas beingnearthe inner tips of the segera'nces which must be'Ladheredto. addition, the operation ot 'the conventional" magnetron jme r a' igm a, couples' 'into 0 1y ne cavity. more power is drawn may become unbalanced because of the unsymt c'fuplingrj 'As the coupling device frointh'at avity than'the others. If it should be desirable to tune the tube by means of an external device,thefi-eld'in one cavity would become distorted to a-greater extent than the fields in the other cavities. Tliusthe field configuraman of jthel inclividu al{cavities would not be such as to produce mammumipower output and emciency. Theefiect oi the impedance'oi the outputcircuit on the frequency of the magnetron is knowninthe j'art' a's pulling, and, it is exasymmetries are usually controlled by adjusting the size of the loopand especially'the impedance of t ep tputeireuitjsb that the pulling figure is suiiicientlylowtojobtain the desired frequency stability together with reasonably high efiiciency. lI Iqwever, "with the asymmetric coupling such solul sion1 'ulsuallythejcase of the best possible L The pres ntinvention'discloses a novel strucltra-high frequency magnetron, and

more particularly a-n'ovel structurefor coupling resonators to an output circuit, structure being symmetrical in respeetfto allr'esonatorsso that each r k equally coupled to'the output a m lgecaiis' oi'thissymmetry of the couplir'ig truc'tur'e', the fieldconfigura-tions also-retain their symmetry the latter phenomenon cont'ributg t' ward "stability 'ofioperation' in the s'c'elcted mode andeflic'iency ofo'peration. Moreover, the previously nentioned impedance of the output circuitj'ma y-now be varied over wider limits sinceyalthoughsuch impedance variation does have the previously'mlentioned effects on of'themag'netronin terms of its We abiiity, 'the limits o f the impedancef variations now 'ybe wider. Accordingly, .su'ch variatiofi in th e' "load impedance mayfbe used now for tuning the magnetron, as will be explained more fully later. Moreover the configuration of the coupling structure, together with some additional elements, is such that it performs a triple function, firstof coupling the resonators to the output circuit, second-of acting as a strapping means for the magnetron resonators to obtain more favorable mode separations and stability of. operation in the selected mode, and .third.the.. anode distributed .reluctances are made to favor the desired pi-mode and block th remaining modes, thus enhancing the mode separation still further.

It is therefore an object of the present invention to provide a symmetrical magnetron out-.

put coupling for connectingv ;a -multiresonator anode of the magnetron to'an output cirouit,.;the coupling being equally coupled to. eachresonator of the anode.

Another object of this invention is to provide tight, low resistance strapping system for a multiresonator magnetron.

An additional object of this invention is to provide a symmetrical magnetron coupling for connecting a multiresonator anode of the magnetron to .an output circuit, this coupling performing a dual function, one of coupling the magnetron to the outgoing line or wave guide, and another of acting as low resistance strapping means for the magnetron resonators to obtain more favorable mode separation, and stability of operation in the selected mode.-

Still another object of this invention is to pro.- vide a symmetrical coupling for a magnetron, which, because of its equal coupling to all resonators, may be'used for external tuning of the output circuit, so that tuning of the magnetron is accomplished by varying theimpedance of the output. circuit and the fpulling. effect exerted by the output circuit impedance on the anode frequency.

Yet another object of this invention is .to provide a multiresonatormagnetronwith wide mode separation between pi-mode and all other modes, this mode separation being obtained by providing preferred, low reluctance paths for magnetic fields due to the pi-mode resonance, and by blocking the distributed reluctance paths for the fields of other modes.

In accordance with the present invention there i provided a magnetron in which the internal structure is so arranged asto provide a common path for the fields of adjacent cavities. In this manner the fields of adjacent cavities are coupled tightly to one another, and the effect is the same as increasing the coupling between alternate cavities, with a concomitant increase in mode separation. In addition there is provided a type of output coupling which is symmetrical with respect to the individual cavities, resulting in increased efficiency and stability pf operation.

These and other features of my invention will be more clearly understood from the following description and theaccompanying drawings, in which:

Fig. 1 illustrates a perspective exploded View of the anode structure andits .outgoing concentric line;

Fig. 2 illustrates a perspective view of the anode and its output coupling line, with their front portion cut away by a vertical plane passing through the axis of the magnetron;

Fig. 3 illustrates a side view of one type of suitable magnetronmounting and connections between the magnetron and one type of output 011:-

cuit;

line GI used Fig. 4 and Fig. 5 illustrate two magnetic flux arrangements available with two possible tube structures;

Fig. 6 illustrates the electric field pattern for both arrangements of magnetic field; and

Fig. '7 illustrates the field pattern of the permanent magnet when the latter has a central bore.

Referring 'nowzto Figs. 1 and 2, the invention is illustrated bythe way of "an example, with a hole-and-slot magnetron which comprises a cylindrical cathode IE9, and a hole-and-slot anode 12. While the invention is illustrated with a hole-and-slotanode, from the description that is to follow it will be obvious to those skilled in the art that the novel structures are equally applicable to .magnetrons having other types of anodes, such as a vane-type anode, or any other type of symmetrical anode surrounding a centrally mounted cathode. The illustrated anode is an eight hole anode having holes l9 and slots 18.

The anode structure also includes bottom plate H, a spider I4 having its legs 15 overlapping alternate anode segments, a spacer plate l6 having its fingers I? overlapping those anode segments that the spider l4 doe not touch, and a top plate 28. An output coaxial line 20 has its center conductor 21 connected to the center of the spider disk' 14 and its outer conductor 22- velectrically connected to the top plate 28 and. thence through the fingers i! of the spacer plate It to alternate anode segments.

. At the bottomof anode block I2, there is incorporated a spacer plate 23 similar to spacer plate [6. Spacers l6 and 23 each effectively lengthen alternate anod segments in the axial direction. Thus spacer 23 and plate I 1 act as low resistance straps for one set of anode segments. These may be either even or' odd segments, as will be described later. If spacer 23 and plate H strap even segments, then the odd segments are strapped by spider l4 and plate 28 at the top.

Referring now to Fig. 2, which is a cutaway portion of the anode block I2 and .the cathode [0, the cathode is shown as being placed symmetrically with respect to the anode segments, the filament. 3i being inside and electrically insulated from the cathode. Spider i4 and spider legs I5, spacer plate 16' .and spacer plate fingers 11, top plate 28, and inner conductor 2| of coaxial line 20 are all shown in their respective positions.

Referring now to Fig. 3, the magnetron 59 is mounted so that thefield from the permanent magnet 5| is parallel to the cathode axis. The

insulated heater leads .52. come up to the magne- The output line 20, shown coaxial for example,

comes out of the magnetron 5D in a direction parallel to the cathode, and so is brought out through a hole 56 in the upper pole piece 62 connected to the permanent magnet 51. One type of external tuningdevice is shown as anexample, andcomprises an elbow joint Bland a length of line 6| in which a plunger 58 is moved along the line by a tuning knob 59 and screw 60, the position of said plunger determining the length of for matching the magnetron to the load.

1 Referring now to the operation of the system, and to Fig. 1 the magnetron is of the multicavity'transit-time type, with the cavities or cavityresonators I9 being-coupled tothe cathodeanode space by means of slots l8. When a high potential is placed on the anode with respect to the cathode, and a strong magnetic field is applied parallel to the cathode axis, oscillations of the electrons occiir within the cathode-anode space, and the energy from these oscillations is coupled to the cavities I9 by means of slots [8. The cavities l9 oscillate in the pi-mode, the ultra-high frequency magnetic field being parallel to the long axes of the cayities. The placeingpath around the anode, this path being illustratedby'a heavy dotted linelflfl This flux therefore travels up through one cavity, over an anode segment, down through an adjacent cavity, through a path formed by an open space between plate I l, spacer 2 3, and the bottom surface of the'anode l2, and up through the following cavity. This flux structure is called serpentine flux because it follows a serpentine path, the

latter being illustrated in Fig. 4. H n

If this staggered arrangement of the spacers l6 and 23 is not used,and the openings in upper spacer l6 are directly above similar openings in the bottom spacer23, the magnetic flux goesup one cavity, down an adjacent cavity and thence back to the first cavity. This flux arrangement is called ring flux, and its path is illustrated by a dotted line 500 in Fig. 5. For both the serpentine fiux and the ring 'fiux, theelectric field in and between the cavities is the same, and is illustrated by lines 600 in Fig; c

,The electric field structure of Fig. 6 is such that if a coaxial line is used for output coupling, the-fundamental mode is excited in theline. If awave guide is used forextracting energy, it may be circular, in which case the TMo,1 mode is propagated in the guide. the electric field in the guide is parallel to the axis, the magnetic field being transverse. Propagation in the guide will have to be facilitated, however, by a probe secured to the top of the spider and extending a slight distance into and along the axis of the guide' In order to bring the coaxial line and heater leads out of the magnetron, holes 54 and 56 are placed in the pole pieces 30 and 62, which are connected to permanent magnet The effect of these holes, referring to Fig. '7, is to increase the magnetic field in the center of the pole pieces. This distortion of the field does not have an undesirable effect upon the operation of the tube.

It has been stated in the objects of the invention that the disclosed output coupling also acts as an effective strapping means for the anode structure. This follows from the electrical connections between the spacers and the anode segments, between the spacers and the upper and lower plates I and 28, respectively, and the connections between the anode segments and the spider. The disclosed method of strapping is more effective electrically, and simpler mechanically, than the known strapping means, which ordinarily comprises two concentric metallic This mode is one in which rings, onefring being connected to the tips of odd resonators, while the other ring is connected to the' ti'ps" of the even resonators. Generally speaking, the effectiveness of any strapping sys- -t muesefiastn the low resistance, or low ultrahigh frequency impedance, of each individual strap. Comparison of the mechanical features f'of the disclosed strapping with the most widely used concentric ring type of strapping reveals the fact that the resistances of the disclosed strapping links will be inherently much lower than the resistance of any ring type of strapping since the'currents due to strapping in the disclosed system"will flow'al'on'g the disks II and 28 or spider [4. Since such paths will have much largersurfaces 'and'shorter lengths than in the ring strapping, they also will have correspondviously in the specification, and it may be recalled that they may have a serpentine or a loop configuration, depending upon the angular positions of the upper and lower spacers with respect to each other, as illustrated in Figs. 4 and 5. In either case, the pi-mode flux is provided with low, uniform reluctance paths throughout the entire corifigurati'onof the anodestructure in both cases. It should be mentioned here that the conventional magnetron, and especially the end spaces aboveand below the anode, also provide uniform low'reluctance paths "for the magnetic fields of theabove mentioned type. However, the known structure does not block the distributed reluctance paths for all other modes, and, as a consequence, all other modes, as a rule, are also providd'with relatively low distributed reluctance path for the magnetic fields generated by them. In the disclosed'structure, while the paths for the magnetic fields due to pi-mode have low i'eluctaance, the paths for all other modes areefi'ectively blocked by the spacers 23 and I6 and covers I l and 28, and especially by the radial extensions." I! of the spacers. Consequently, the

magnetic fields of all other modes would encounter only high distributed reluctance paths. Such configuration of the reluctance paths for various modes, as it has been demonstrated by theoretical analysis of the anode structure of this type and by subsequent actual operation of the magnetrons constructed in accordance with the teachings of this invention, produces increased separation of the desired pi-mode from all other parasitic modes.

While there has been described what is at present considered the preferred embodiment of the invent-ion, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications a fall within the true spirit and scope of the invention.

I claim:

1. An ultra-high frequency magnetron multiresonator anode with a plurality of radially dis- 7 nosed n de s sments th es n ors t s a od -bein de n n r y a d s ment and metallic plates at-both ends of said anode defining serpentine paths forthe ultra-high frequency magnetic field generated in said resonators when said magnetron is operated in the pi mode.

2. An ultra-high frequency magnetron multir-esonator anode with a. plurality of radially dis.- posed anode Segments, the resonators of said anode being defined in part by said segments, and a pair of metallic plate elements respectively connected to-different alternate segments, said elements being respectively disposed at different ends of said anode, said resonators defining loop paths for the ultra-high frequency magnetic fields generated in said resonators when said magnetron is operated in the pi mode, each of saidloop paths being down through one resonator,-u-p through adjacent resonator, and across the respective ends of the anode segment separating the downwardand upward paths of said fields.

3. An ultra-high frequency multiresonatoranode with a plurality of anode segments, the even segments of said anode being higher than the odd segments at, the upper end of said anode, and the odd segments of said anode being higher than the even segments at the lower end of said anode, a first metallic plate resting on said even segments only at the upper end of said anode, and a second metallic platemounted upon the odd segments only at the lower end of said anode, whereby the ultra-high frequency magnetic field generated in said resonators follow a serpentine path when said magnetron is operated in the pi mode.

4. an ultra-high frequency multiresonator cylindrical anode, having upper and lower plane surfaces with a plurality of anode segments positioned between said upperand lower surfaces, a first spacer mounted on said upper surface, said first spacer having extensions resting on the even segments only. of said anode, a first end-plate mounted on. said first spacer, a second spacer mounted on said lower surface, said second spacer having extensions resting on the odd segments only of said anode, and a second end-plate mounted onsaid; second spacer.

5, An ultra-high frequency magnetron multiresonator anode with a plurality of anode segments, the odd segments of said anode being 8 ing its inner conductor connected to the even se ments only.

6. An ultra highfrequency magnetron multiresonator anode having upper and lower plane surfaces with a plurality of anode segments positioned between said upper and lower surfaces, a first; spacer mounted on said upper surface, said first spacer having extensions resting on the even segm nts only of said anode, a first end-plate mounted on said first pacer, a second spacer mounted, on said lower surface, said second spacerhaving extensions resting on the even segmentsonly of said anode, and a second end-plate mounted on said second spacer.

7. A magnetron multi-resonator anode having upper and lower plane surfaces with a plurality of anode segments positioned between said upper and lower surfaces, a first spacer mounted on said upper surface, said first spacer having extensions resting on alternate segments only of said anode, a first end plate mounted on said first spacer, a second spacer mounted on said lower surface, said second spacerhaving extensions resting on alternatesegment only of said anode, and a secand endplate mounted on the second spacer.

8. A strapped-magnetron comprising a cathode, a multi-resonator anodewith a plurality of radially disposed anode segments, and two spaced discs contiguous-t0 saidseg-ments for strapping said segments, one disc being connected to the even anode segments only, and the other disc being connected to theoddanode segments only, said discs being coaxiallymounted with the axis of said anode, the locus of the outer axial surface of said anode being a cylinder and the diameter of the disc connected to the even anode segments being substantially equal to the diameter of said cylinder.

PRESCOTT D. GROUT.

References Cited in the file of this patent UNITED STATES PATENTS 

